adding djfft as another fft implementation

[brightening-eyes]

hello,
djfft is available under unlicense and mit. unlike pfft which is under gpl, this one can be linked statically and is a single header only lib.
it has implementations for GPU as well.

[achandak]

@brightening-eyes - We use PFFFT, which has its own license terms and is NOT under GPL. https://github.com/ValveSoftware/steam-audio/blob/master/core/THIRDPARTY.md#pffft

Here is the repo for the project. https://github.com/marton78/pffft